Field of the Invention
The invention relates to an optical reader including a multi-channel detector.
Various readers and optical scanning systems have been developed for reading printed indicia such as bar code symbols appearing on a label or the surface of an article and providing information concerning the article such as the price or nature of the article. The bar code symbol itself is a coded pattern of indicia comprised of, for example, a series of bars of various widths spaced apart from one another to form spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals which are decoded into alpha-numeric characters that are intended to be descriptive of the article or a characteristic thereof. Such characters typically are represented in digital form, and utilised as an input to a data processing system for applications in point of sale processing, inventory control and the like.
Known scanning systems comprise a light source for generating a light beam incident on a bar code symbol and a light receiver for receiving the reflected light and decoding the information contained in the bar code symbol accordingly. The readers may comprise a flying spot scanning system wherein the light beam is scanned rapidly across a bar code symbol to be read or a fixed field of view reading system wherein the bar code symbol to be read is illuminated as a whole and a CCD (Charge Coupled Device) array is provided for detecting the light reflected from the bar code symbol. The reader may be either a hand-held device or a surface-mounted fixed terminal.
A variety of scanning devices are known. The scanner could be a wand type reader including an emitter and a detector fixedly mounted in the wand, in which case the user manually moves the wand across the symbol. Alternatively, an optical scanner scans a light beam such as a laser beam across the symbol, and a detector senses the light reflected from the symbol. Alternatively a gun-type hand-held arrangement may be provided. In either case, the detector senses reflected light from a spot scanned across the symbol, and the detector provides the analog scan signal representing the encoded information.
A digitizer processes the analog signal to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The threshold level effectively defines what portions of a signal the reader will recognize as a bar or a space.
Readers of the type discussed above are single channel systems having a single digitizer output and/or a single processing chain to produce a single digitized output.
The pulse signal from the digitizer is applied to a decoder. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyses the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. The recognition of the standard is typically referred to as auto discrimination.
Different bar codes have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on a appropriate medium is exacting and thus is more expensive than printing low resolution symbols.
A bar code reader typically will have a specified resolution, often expressed by the size of its effective sensing spot. The resolution of the reader is established by parameters of the emitter or the detector, by lenses or apertures associated with either the emitter or the detector, by the threshold level of the digitizer, by programming in the decoder, or by a combination of two or more of these elements.
In a laser beam scanner the effective sensing spot may correspond to the size of the beam at the point it impinges on the bar code. In a scanner using an LED or the like, the spot size can be the illuminated area, or the spot size can be that portion of the illuminated area from which the detector effectively senses light reflections. By what ever means the spot size is set for a particular reader, the photodetector will effectively average the light detected over the area of the sensing spot.
A high resolution reader has a small spot size and can decode high density symbols. The high resolution reader, however, may have trouble accurately reading low density symbols because of the lower quality printing used for such symbols. This is particularly true of dot matrix type printed symbols. The high resolution reader may actually sense dot widths within a bar as individual bar elements. In contrast, a low resolution reader has a large spot size and can decode low density symbols. However, a reader for relatively noisy symbols such as dot matrix symbols reads such a wide spot that two or more fine bars of a high resolution symbol may be within the spot at the same time. Consequently, a reader having a low resolution, compatible with dot matrix symbols can not accurately read high density symbols. Thus any reader having a fixed resolution will be capable of reading bar codes only within a limited range of symbol densities.
For a given symbol density, the resolution of the reader also limits the range of the working angle, i.e. the angle between the axis of the reader and a line normal to of the surface on which the bar code is printed. If the range and resolution are too limited, a user may have difficulty holding a hand-held reader comfortably while accurately scanning the bar code. This can be particularly troublesome if the reader incorporates additional elements to form an integrated data terminal. The combination of size, weight and an uncomfortable angle can make reading in large amounts of bar code information difficult and annoying, and thereby make the user more resistant to use of the bar code system. Problems also arise with fixed readers which may have to be manually switched dependent on the resolution required, reducing efficiency and slowing operation of the system.
One solution might be to provide some means to adjust the resolution or sensing spot size of the reader, e.g., by adjusting the threshold of the digitizer. This approach, however, would require a number of different scans at different resolutions. If the scan is automatic, the variation in resolution causes a loss of robustness because the scan is at the correct resolution only a reduced amount of the time. Effectively such a scanner would scan at the equivalent of a reduced rate. If the reader is a hand-held device, the user would have to manually scan the reader across the information each time the resolution changes. This causes a marked reduction in the first read rate and increased frustration for the user.
In addition most optical scanners such as bar code scanners are adapted for use at a particular distance, or a range of distances, from an indicia to be scanned. If the user holds the scanner too close to the indicia, (or, conversely the object is held too close to the scanner) or too far away, the indicia and/or the flying spot beam will not be in focus, and decoded will not be possible.
Such scanners may not be particularly convenient in environments where a series of indicia to be read are presented to the scanner at various distances, and where it is difficult or impossible for the user to alter the distance between the scanner and the indicia. To deal with such situations, attempts have been made to expand the acceptable working range of conventional scanners, to give the user as much leeway as possible, and also to provide multi-distance scanners which can operate, for example, at a first working range or at a second working range according to the user's preference or requirements. One possibility is for the provision of a two-position switch on the scanner, with the scanner operating at a first working distance in a first position of the switch and at a second working distance in a second position.
An improved solution is to provide an optical scanner including a multi-surface reflector having a first surface of a first profile and a second surface of a second profile, the scanner being adapted to read an indicia at a first distance or working range when the beam is reflected from the first surface and at a second distance or working range when the beam is reflected from the second surface.
A further problem associated with conventional methods of signal processing is that the analog signal is processed only once and the decodeability of the processed signal is principally determined by the signal to noise ratio of the incoming signal.
In addition known digitizers have been recognised as requiring improvement in the scanning of dot matrix symbols such as bar code symbols printed in dot matrix format. The failure to decode such symbols is commonly caused by a split in a wide bar wherein a void in the bar causes a narrow element to appear in the middle of the bar. This only happens in the case of wide bars because such bars are built up of two or three columns of dots and small spaces between the columns are sometimes large enough to be recognised and digitized as wide elements.
It will be appreciated that the problem does not arise with narrow bars made up a single column of dots.